This invention relates to a trona crystallization process and more particularly this invention relates to a double crystallization process to produce low organic, low silica anhydrous sodium carbonate.
Trona as found in the Green River area of Wyoming consists mainly of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2 H.sub.2 O). A typical analysis of trona contains
______________________________________ Constituent Weight Percent ______________________________________ Na.sub.2 CO.sub.3 45 NaHCO.sub.3 36 H.sub.2 O 15.30 NaCl 0.04 Na.sub.2 SO.sub.4 0.01 Fe.sub.2 O.sub.3 0.14 Organic Matter 0.30 Insolubles 3.20 ______________________________________
In addition to the water insoluble fraction resulting from the association of the trona with shale in the trona deposits, organic matter in the order of about 0.3 percent is present which would contaminate the desired product, e.g., sodium carbonate precursor crystals, unless it is removed. It is believed that the organic matter in trona consists of kerogenaceous material containing monocarboxylic acids, dicarboxylic acids, certain unsaturated acids steroids and certain rosin acids.
Furthermore, in order to improve the desired physical properties of the sodium carbonate precursor crystals, it is customary to add to the solution to be crystallized, organic surface active agents, such as alkylated benzene sulfonate, as crystallization modifiers or foam depressers to improve crystallization. Likewise, organic defoaming agents and other organic impurities picked up in the circulating process liquors in the plant are present in the solution prior to and during crystallization. The presence of these organics to any appreciable extent is not desired because they adversely affect the crystal quality, e.g., discoloration of the crystal and reduction in bulk density, which may limit the extent of use of the precursor crystals, particularly if they are to be converted to dense soda ash for use in the production of sodium tripolyphosphate. Various methods previously have been proposed to reduce organics to an acceptable level so as not to adversely affect the crystal quality. For instance, in U.S. Pat. Nos. 2,962,348 and 3,131,996 the crude trona is decarbonized at relatively high temperatures. These patents disclose that at temperatures in the order of about 400.degree. to 800.degree. C., the organic matter present in the crude trona is substantially removed as it is converted to a less soluble state which, as such, is removed in the subsequent steps of dissolving, clarifying and filtration prior to crystallization. This method, of course, would involve a high heat requirement, thus increasing the cost of the ultimate product. Also, heating the trona in excess of 400.degree. C., while eliminating substantial amounts of the carbon in the crude trona, substantially increases the quantity of water-soluble silica in the crude sodium carbonate, the removal of which is difficult, requiring the bleeding off of substantial amounts of the mother liquor separated from crystallizers. This bleeding off results in the loss of soluble carbonate values in the discarded mother liquor.
Another method proposed is that described in U.S. Pat. No. 3,028,215, wherein the sodium carbonate precursor containing organic impurities is subjected to a high temperature calcination, i.e., temperatures in the order of about 400.degree. C. to 800.degree. C., wherein the organic impurities are either volatilized or converted to a coke which allegedly does not present any problem in the utilization of the resulting soda ash in commerce. However, the crystal quality is not significantly improved because the organics are present during crystallization at which time their adverse effect is realized on the quality of the crystal produced during the crystallization. Also, the equipment and heat requirements to calcine the contaminated sodium carbonate precursor crystals are significant, particularly when the production rate is in the magnitude of more than 1,000 tons of product per day.
Still another method for reducing organics is that described in U.S. Pat. No. 3,260,567, wherein the crude trona is first converted to crude sodium carbonate at relatively low temperatures, i.e., in the order of about 150.degree. to 200.degree. C., and then the crude sodium carbonate is dissolved in aqueous solution. After removal of the solubles by clarification, the organics are substantially reduced by passing a solution of crude sodium carbonate through a bed of adsorbent, such as activated carbon, prior to crystallization. After crystallization the mother liquors are recycled through the adsorbent beds to reduce the build up of organics in the mother liquor streams prior to a second crystallization. While there is little, if any, necessity to bleed off any mother liquor due to organic build up in the recycled mother liquor, special equipment is necessary to reactivate the large amounts of the activated carbon when it no longer effectively performs its function. Also, there is needed to effect this operation an increase in the necessary processing equipment due to the large volumes of liquid, i.e., fresh feed solution as well as recycled mother liquor to be handled.
Other trona crystallization processes are described in U.S. Pat. Nos. 3,653,848, and 3,705,790.
While these patents disclose crystallization processes to produce soda ash, all involve elaborate processing techniques which require substantial cost and capital equipment, energy, additives and consumption of time. Thus, there still is a need for a simple, low energy process to convert impure trona ore containing soluble and insoluble impurities into pure soda ash. A low organic, low silica soda ash product would be particularly desirable.
Accordingly, it is an object of the present invention to provide a low energy process which produces low organic, low silica soda ash without employing chemical additives or foam depressors.